Recently, as electrolytes for lithium ion secondary batteries, nonaqueous electrolytes, comprising a nonaqueous electrolyte compound, a lithium salt, and if necessary, other additives, have been used.
The nonaqueous electrolyte compound can generally consist of a combination of cyclic carbonate and linear carbonate. Examples of the cyclic carbonate include ethylene carbonate (EC), propylene carbonate (PC), and gamma-butyrolactone (GBL), and examples of the linear carbonate include diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC).
The lithium salt is used to supply lithium ions to electrolytes, and other additives are selectively used to improve the performance of electrolytes and batteries.
In the nonaqueous electrolyte lithium secondary battery, the safety of the battery in an overcharged state generally becomes the greatest problem. Among the causes of the safety problem, an important cause is an exothermic reaction resulting from structural degradation of the cathode. The exothermic reaction occurs based on the following principles.
A cathode material consisting of, for example, lithium-containing metal oxide capable of absorbing and releasing lithium and/or lithium ions, is changed into a thermodynamically unstable structure as a result of the deintercalation of lithium ions when the battery is overcharged. When the temperature of the battery in this overcharged state reaches the critical temperature due to external physical impact, for example, exposure to high temperatures, oxygen will be released from the cathode material having the unstable structure. The released oxygen will cause an exothermic decomposition reaction with, for example, an electrolyte solvent, and the exothermic decomposition of the electrolyte, caused by this reaction, will be accelerated by oxygen released from the cathode. Due to such successive exothermic decomposition reactions, the battery will undergo thermal runaway, leading to ignition and explosion.
In attempts to control the above-described ignition or explosion resulting from the temperature rise in the battery, many solutions have been suggested, and one example thereof is a method that uses additives (nonaqueous electrolyte additives). As the nonaqueous electrolyte additives, additives are known, which use redox shuttle mechanisms, for example, chloroanisole, and additives that use polymerization mechanisms, for example, alkylbenzene derivatives, such as cyclohexylbenzene, and biphenyl.
Specifically, as an additive for improving the safety of a battery upon overcharge of the battery, a material undergoing oxidation-reduction cycling, for example, chloroanisole, is sometimes used, but this material has a problem in that it is not effective when the charge current of the battery is high. As another method, there is a method in which the monomer of a conductive polymer, such as biphenyl that can block the flow of electric current by forming a blocking layer through the polymerization thereof upon overcharge of the battery, is added to an electrolyte. However, in the case where the monomer of the conductive polymer, such as biphenyl, is used, there are problems in that the performance of the battery will be deteriorated due to an increase in resistance, and the monomer should be used in a large amount to ensure sufficient safety.